Magnetoplasmadynamics



Oct. 18, 1966 T. 1.. ROSEBROCK 3,279,320

MAGNETOPLASMADYNAMICS Filed May 29, 1964 HEATING CUR R EN T SOURCE if 67 IN VENTOR.

LOA D fiwaozz Z; %sefirock ATTORNEY United States Patent 3,279,320 MAGNETOPLASMADYNAMICS Theodore L. Rosebrock, Indianapolis, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed May 29, 1964, Ser. No. 371,220 12 Claims. (Cl. 89-8) This invention relates to an improved method and apparatus for generating and utilizing a plasma.

The elfective adaption of the magnetoplasmadyna-mic principle to useful purpose, such as space propulsion and the generation of electricity, has been seriously hampered by the inability of known techniques to obtain satisfactory operating efficiencies. For instance, the need to produce successive discharges of stored electrical energy requires a triggering device. In the past this triggering has been done externally, e.g., by a spark gap switch, but this presents a serious problem for as much as half or more of the initial stored electrical energy is lost. Then too erratic dispersion of the plasma, such as occurs with the exploding wire technique, wastes considerable energy.

The new method and apparatus new proposed overcomes these problems by utilizing the plasma generating medium itself to produce the switching. The unique apparatus facilitates the achievement of a unidirectional explosion while at the same time obtaining a high propellant mass utilization.

' The foregoing and other objects and advantages of the invention will be apparent from the following description and from the accompanying drawings, in which:

FIGURE 1 is a schematic illustration of magnetoplasmadynamic apparatus incorporating the principles of the invention; and

FIGURE 2 illustrates schematically a generator for use in conjunction with the FIGURE 1 apparatus.

Referring first to the FIGURE 1 apparatus, the numeral denotes generally an electromagnetic plasma propulsion device or accelerator; the numeral 12 an electrical energy source; and the numeral 14 an electrically conductive medium or propellant source. As will become apparent the propellant from the source 14 acts as a switch for the application of electrical energy from the source 14 to the accelerator 1 0. The resultant firing of the accelerator 10 generates from the propellant a plasma having a very high unidirectional velocity component.

Considering first the structure of the accelerator 10, which is of the rail type, a pair of electrodes 16 and 18 are provided and are made from a material that will provide maximum contact velocity while avoiding electrode evaporation. By way of example, silver Elkonite, Mallory 1,000 or some similar alloy may be used. The electrodes 16 and 18 have substantially parallel inner surfaces 20 spaced apart so as to form a gap. At the opposite end the electrodes have cutouts at 22 for reception of a firing surface 24 formed on the periphery of a transport wheel 26. In the vicinity of the cutout 22 electrodes 16 and 18 each have diverging surfaces 28 approximately normal to the periphery of the firing surface 14. These diverging surfaces 18 minimize the impingement of the exploding vapor on the inner surfaces of the electrodes 16 and 18. The electrodes 16 and 18 are connected to a coax at 30 and are electrically insulated from each other by an appropriate strip of material 32. This arrangement is merely one way of achieving a low inductance coupling to the capacitor 54. Others will readily occur to those skilled in the art.

The transport wheel 26 is necessarily made of an electrically inert material, e.g., high strength glass, boron nitride, lava, etc. Support for the wheel 26 is provided ice by two idler rollers 34 and 36 and a drive roller 38. One of the idler rollers 34 or 36 may be spring loaded in a known way to maintain this three-point contact without binding. The drive roller 38 may be revolved by a suitable motor 39. In this embodiment a. variable speed electric motor is used. The firing surface .24 of the wheel 26 is made slightly concave to give the exploding plasma a small initial velocity component toward the plane of the electrodes 16 and 18 so as to aid in the confinement of the plasma to be deposited thereon by the propellant source 14.

The propellant source 14 is essentially a boiler capable of operating at a very high temperature :and comprises a thin walled tantalum tube 40 shaped to have an exit orifice adjacent the firing surface 24 on the transport wheel 26. The shape of the orifice is preferably rectangular with one side parallel to the direction of motion of the transport wheel 26. The boiler tube 40 is electrically heated by its own resistance, the required heating current may 'be derived from any conventional heating current source 42. The tube 40 serves as a center conductor by being maintained within a bracket 44 that at its lower end is provided with a coax 46, thus providing a complete circuit for the heater current while avoiding the generation of any significant magnetic field in the vicinity of the accelerator 10. The boiler tube 40 is properly, thermally insulated and surrounded by some type of polished metal heat shield, not shown, so as to minimize heat losses.

The aforedescribed boiler, of course, is merely exemplary. Actually, any arrangement that will provide the necessary vapor temperatures can be used.

Silver, cadmium, copper, Nichrome, Zinc, and bismuth are exemplary propellants. Silver has excellent vacuum plating characteristics, but cadmium has lower vaporization energy and higher vapor pressure, and therefore permits operation at a lower boiler temperature. With orifice areas between 10 and 10- in. silver requires boiler temperature bet-ween 1200 and 1400 K.; whereas cadmium requires a temperature of between 520 and 570 K.

The electrical energy source 12 includes a DC. source 48 that is connected to the coax 30 through a resistor 50 and an inductor 52. Energy storage is attained by the parallel capacitor 54. The characteristics of the capacitor 54 will be determined by the charging voltages, which will be in the approximate range of 2500 to 5000 volts, keeping in mind that the highest voltages are generally used When operating at the higher plasma Velocities. With the source 48 having a 30 kw. power level, a capacitor of about 60 microfarads was used. A 30 microfarad capacitor has also operated satisfactorily. The coax 30 affords the needed minimum inductance path to the electrodes 16 and 18. The values of the resistor 50, the inductor 52 and the capacitor 54 all, of course, alTect the charging time constant of the circuit and the charging efiiciency; whereas the value of the capacitor 54 will influence the discharge time constant.

Describing now the operation and keeping in mind that the operation takes place in a vacuum environment, the boiler tube 40 will, as the transport wheel 26 revolves, apply the propellant in vapor form to the firing surface 24, which as has been mentioned in electrically inert and which will preferably revolve at a constant rpm. at any one setting. It is mentioned here that by being able to vary the speed of the motor 39 and accordingly the wheel speed and also the boiler operating temperatures, the plasma exit velocity can be accurately controlled. With the previously suggested orifice areas approximately a one centimeter wide strip of the propellant will be applied to the firing surface 24 where it will condense and almost immediately solidify. The solid propellant will be rotated into the gap of about 4 cms. between the electrodes 3 16 and 18 and complete the circuit extending from between the electrodes 16 and 18 to the charged capacitor 54. Upon completion of this circuit an electrical discharge Will occur with current flowing, for exemplary urposes, in the direction of the arrows, i.e., from the electrode 18 through the solidified propellant and then through the electrode 16. The resultant magnetic fields will be as portrayed by the circular arrows, and as will be noted are cumulative, i.e., one supplements the other. The capacitor discharge electrically explodes the propellant producing a plasma having an initial unidirectional velocity in the direction of the arrow 56, this being facilitated by the diverging surfaces 28 and the concave shape of the firing surface 24. The plasma is further accelerated by these magnetic fields and will achieve very high velocities; e.g'., 240,000 'miles per hour; As will be appreciated,

the unidirectional explosion in the direction of arrow 56 avoids the usual plasma dispersion experienced with wire explosions. The now scavenged firing surface 24 between the electrodes 16 and 18 permits the capacitor 54 to be recharged until the next segment of the propellant is moved into the gap. Thus the frequency of the discharges will be controlled by the speed at which the transport wheel 26 is revolved. With a 20 cm. wheel diameter and a wheel speed of 380 r.p.m., a firing pulse rate of approximately 100 pulses per second will be achieved.

The very high velocities of the plasma afford either engine or generator operation. As an engine, of course, the high velocities provide a thrust force for a driving agency. To perform as a generator, the high velocity plasma may be transferred through a suitable channel, such as displayed in FIGURE 2, defined by a pair of spaced electrodes 58 and 60, and the faces of a magnet 62, which produces a magnetic field that is perpendicular to the direction of the plasma flow. This develops a potential across the electrodes 58 and 60 and a direct current is produced that may be utilized by a load 64.

. As will now be appreciated, the invention provides electroplasmadynamic apparatus with an electromagnetic accelerator that is repetitively pulsed by the continuous vacuum deposition of a thin propellant film on a transport wheel and serves as a switching or trigger agency for the discharges. This eliminates the need for any kind of a propellant valve or external switch and all the usual controls. Moreover, virtually all of the propellant is utilized and dispersion of the plasma is controlled so as to give a unidirectional velocity component. More simply stated, the propellant itself acts as a switch and the firing rate is easily regulated by the speed of the rotating transport wheel 26.

Various values and material have been suggested throughout this description. These are for demonstration purposes only and should not be construed as in any way restricting the invention, which is to be limited only by the following claims. Obviously, if optimum results are to be achieved to meet different operating conditions, changes will be required as those versed in the art will appreciate.

What is claimed is:

1. In magnetoplasmadynamic apparatus, the combination of a maneuverable member having a firing surface thereon for an electrically conductive material, a pair of electrode surfaces arranged in close proximity to the'firing surface and extending therefrom in substantially parallel relation so as to form a gap therebetween, means Inaneuvering the firing surface of the maneuverable memher through the gap at a certain cyclic rate, means applying the electrically conductive material to the firing surface, and means applying a voltage across the gap so that each time the material bridges the gap an electrical discharge occurs forming from the material a plasma that is accelerated and ejected from between the electrode surfaces, the electrical discharges occurring at a rate corresponding to the certain cyclic rate.

2. In magnetoplasmadynamic apparatus, the combination of a movable transport member formed of an electrically non-conductive material and having a concave firing surface, a pair of electrode surfaces arranged in close proximity to the firing surface and extending therefrom in substantially parallel relation so as to form a gap therebetween, means maneuvering the firing surface of the transport member through the gap at a certain cyclic rate, means applying an electrically conductive material to the firing surface, and means applying a voltage across the gap so that each time the electrically conductive material bridges the gap an electrical discharge occurs forming from the electrically conductive material a plasma that is accelerated and ejected from between. the electrode surfaces, the electrical discharges occurring at a rate corresponding to the certain cyclic rate.

' 3. In magnetoplasmadynamic apparatus adapted for operation in a vacuum environment, the combination of a revolvable transport wheel formed of a nonconductive material and having a peripheral concave firing surface, means revolving the transport wheel at a certain cyclic rate, a pair of electrodes each having diverging surfaces in close proximity to the firing surface of the transport wheel and substantially parallel surfaces spaced apart so as to form a gap therebetween, means continuously applying an electrically conductive medium to the peripheral concave firing surface, and means applying a voltage across the gap so that each time the medium bridges the gap an electrical discharge occurs through the medium thereby forming from the medium a plasma that is accelerated and ejected from between the electrodes.

4. In magnetoplasmadynamic apparatus, the combination of a maneuverable transport member formed of an electrically nonconductive material and having a firing surface, a pair of electrodes having the ends thereof insulated from each other and in close proximity to the firing surface, the electrodes each extending outwardly from the ends thereof in a direction substantially normal to the firing surface and then extending substantially parallel to each other so as to provide a certain gap therebetween, means maneuvering the firing surface through the gap at a certain cyclic rate, means continuously applying an electrically conductive medium to the firing surface, and electrical energy storage means connected across the gap so that each time the medium bridges the gap an electrical dis-v charge occurs forming from the medium a plasma that is accelerated and ejected from between the electrodes due to the action of the electrical discharge with the selfinduced magnetic field, the electrical discharges occurring at a rate corresponding to the certain cyclic rate.

5. In magnetoplasmadynamic apparatus, the combination of a revolvable transport wheel formed of an electrically nonconductive material and having a groove in the periphery thereof, a pair of electrodes having the ends thereof insulated from each other and in close proximity to the periphery of the transport wheel, the electrodes each extending outwardly from the ends thereof in a direction substantially normal to the periphery of the transport wheel and then extending substantially parallel to each other so as to provide a certain gap therebetween, means revolving the transport wheel at a certain speed, means continuously applying an electrically conductive medium to the groove of the transport wheel, and voltage storage means connected across the gap so that each time the medium bridges the gap an electrical discharge occurs forming from the medium a plasma that is accelerated and ejected from between the electrodes due to the action of the electrical discharge with the self-induced magnetic field.

6. In magnetoplasmadynamic apparatus, the combination of a maneuverable member having a firing surface formed thereon of an electrically nonconductive material, a pair of electrodes arranged in close proximity to the firing surface and spaced apart so as to provide a gap therebetween, means maneuvering the firing surface of the ve able member through the gap at a certain cyclic EB rate, means continuously applying an electrically conductive vapor to the firing surface, and voltage storage means connected across the gap so that each time the vapor bridges the gap an electrical discharge occurs forming a plasma therefrom that is accelerated and ejected from between the electrode surfaces, the electrical discharges occurring at a rate corresponding to the certain cyclic rate.

7. In magnetoplasmadynamic apparatus, the combinaof a pair of electrodes spaced apart within a vacuum environment so as to form a gap therebetween, means applying a voltage across the gap, and switching means producing electrical discharges across the gap, the switching means including a m-aneuverable member having a firing surface arranged in bridging relation with the gap, means maneuvering the firing surface of the maneuverable member through the gap at a predetermined cyclic rate reflecting a desired electrical discharge frequency, means applying an electrically conductive medium to the firing surface so as to close the gap when the firing surface is maneuvered and produce the electrical discharge, thereby forming from the medium a plasma that is accelerated and ejected away from the electrodes, the electrical discharges occurring at the desired electrical discharge frequency.

8. In magnetoplasmadynamic apparatus, the combination of a pair of electrodes spaced apart within a vacuum environment so as to form a gap therebetween, means applying a voltage across the gap, and switching means producing electrical discharges across the gap at a certain frequency, switching means including a movably mounted transport member formed of an electrically nonconductive material and having a con-cave firing surface in bridging relation with the gap, means maneuvering the firing surface of the member through the gap at a predetermined cyclic rate corresponding to the certain frequency, means applying an electrically conductive medium to the concave surface of the member so as to close the gap when the member is maneuvered and produce the electrical discharge thereby causing an explosion so as to form from the medium a plasma having a high velocity unidirectional component.

9. In magnetoplasmadynamic apparatus, the combination of a pair of electrodes arranged within a vacuum environment, the electrodes having the ends of each in close and insulated proximity and so shaped as to each diverge and then extend in substantially parallel relation so as to form a gap therebetween, means applying a voltage across the gap, and switch means producing electrical discharges across the gap at a certain frequency, switching means including a revolvably mounted transport wheel formed of electrically nonconductive material and having a peripheral concave firing surface arranged in bridging relation with the gap, means revolving the wheel at a predetermined cyclic rate corresponding to the certain frequency, means continuously applying to the firing surface a vapor of an electrically conductive material that solidifies so that as the wheel revolves the solid material will close the gap and produce an electrical discharge thereby causing an explosion to form from the material a plasma that is accelerated and ejected away from the electrodes due to the action of the electrical discharge and the self-induced magnetic field.

10. In magnetoplasmadynamic appartus, the combination of a pair of electrodes spaced apart so as to form a gap therebetween, means applying a voltage across the gap, and switch means producing electrical discharges across the gap at a certain frequency, the switching means including a maneuverable member having a firing surface thereon arranged in bridging relation with the gap, means maneuvering the firing surface of the maneuverable member through the gap at a cyclic rate corresponding to the certain frequency, a vapor source of electrically conductive material including a restricted outlet positioned adjacent to the firing surface so as to cause the vapor to be deposited thereon, the vapor solidifying so as to close the gap when the firing surface is maneuvered and produce the electrical discharge, the electrical discharge causing the solid material to form a plasma that is accelerated and ejected away from the electrodes at a high velocity and means utilizing the high velocity plasma.

11. The method of producing a high velocity plasma comprising the steps of developing an electrically conductive propellant vapor, applying the propellant vapor to a movable member having a firing surface thereon and moving the firing surface of the movable member so that the propellant bridges a high voltage source connected gap at a certain cyclic rate and produces electrical explosions at a frequency corresponding to the certain cyclic rate for forming from the propellant a plasma having a high velocity component.

12. The method of producing a high velocity plasma comprising the steps of applying a high voltage to a gap between two rail type electrodes, continuously maneuvering a member having a firing surface thereon between the electrodes at a certain cyclic rate, and applying an electrically conductive medium to the firing surface for bridging the gap and producing an electrical explosion that will form from the medium a plasma having a high velocity unidirectional component, the electrical explosions occurring at a frequency corresponding to the certain cyclic rate.

References Cited by the Examiner UNITED STATES PATENTS 2,975,332 3/1961 Starr 315-169 OTHER REFERENCES NASA Research on Plasma Accelerators: by W. E. Moeckel and W. D. Rayle, presented at the ARS 14th annual meeting November 16, 1959, pp. 1-14.

BENJAMIN A. BORCHELT, Primary Examiner.

SAMUEL W. ENGLE, Examiner. 

1. IN MAGNETOPLASMADYNAMIC APPARATUS, THE COMBINATION OF A MANEUVERABLE MEMBER HAVING A FIRING SURFACE THEREON FOR AN ELECTRICALLY CONDUCTIVE MATERIAL, A PAIR OF ELECTRODE SURFACES ARRANGED IN CLOSE PROXIMITY TO THE FIRING SURFACE AND EXTENDING THEREFROM IN SUBSTANTIALLY PARALLEL RELATION SO AS TO FORM A GAP THEREBETWEEN, MEANS MANEUVERING THE FIRING SURFACE OF THE MANEUVERABLE MEMBER THROUGH THE GAP AT A CERTAIN CYCLIC RATE, MEANS APPLYING THE ELECTRICALLY CONDUCTIVE MATERIAL TO THE FIRING SURFACE, AND MEANS APPLYING A VOLTAGE ACROSS THE GAP SO THAT EACH TIME THE MATERIAL BRIDGES THE GAP AN ELECTRICAL DISCHARGE OCCURS FORMING FROM THE MATERIAL IN PLASMA THAT IS ACCELERATED AND EJECTED FROM BETWEEN THE ELECTRODE SURFACES, THE ELECTRICAL DISCHARGES OCCURRING AT A RATE CORRESPONDING TO THE CERTAIN CYCLIC RATE. 